Electrophotographic marking is a well-known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The charge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
While the REaD IOI process is beneficial it is not without problems. One set of problems relates to transferring the composite color image onto a substrate. Toner transfer is complicated because the REaD IOI process produces a composite toner layers having highly variable charge magnitudes and distributions. For example, at the surface of the composite toner layer some of the toner might have a very low, possible even an opposite polarity, charge while some toner might have a high charge magnitude. These variations, which are likely a result of the recharging steps, produce toner surface potentials that are widely variable. Model projections suggest that the problem increases with the number of recharge steps a toner layer received.
A prior art approach to improving transfer is to use a DC biased corona device to add right-sign charge to the top of a composite toner layer comprised of multiple and or single toner layers. While this approach is promising, it is difficult to create the proper charge distribution in the various toner layers simultaneously. Achieving a charge distribution that provides good transfer for some of the toner (say a black toner layer) creates a poor distribution degrading transfer for some of the remaining toner (say a cyan or composite blue layers). Therefore, a new approach for simultaneously optimizing the charge distributions in single and multiple composite layers for transfer would be beneficial.